I recently started playing around on LOTR Online, just to give it a chance (it's also free). You can buy stuff for it, just like any free MMO - but it's not necessary. If you have the settings - that game actually looks great too, but make sure you are a fan of Tolkien and fantasy because it's high roleplay and narrative. Wall of text quest-givers. Even if you are just going out to kill bugs for them.
League of Legends is another free game, DOTA-style, that can just be plain fun.
But to be perfectly honest, THEY ARE ALL TIME SINKS. MMO's take up a lot of your time, even the free ones. I know, I've played W.o.W since it first came out, but I learned to actually balance everything and get things done during the day and see friends and family, but it was difficult. I did not think it would take that much to learn to STOP playing it so much. Though I've recently taken a break until Cataclysm. The shine has worn off the game after so long, if you will. I still enjoy it, but, I don't feel like I HAVE to play it. After a couple years, I finally learned how to be a casual player on that game. Some are better at being a casual player, then others.
Even League of Legends can sink your time. Every 5v5 match is 45-60 minutes, and they can go over that time if the teams are equally matched and know what they are doing. People are playing one match one right after the other, tells you how much time they have. But damn it, it's fun! =D
I've actually found that Eve online, isn't as bad as the rest because skills take so long to learn. Days even. However, Eve online can also be a pain in the ass, and has a steep learning curve. I was just getting the hang of it, as soon as my 14 day trial EXPIRED. But it's one hell of a beautiful game. I mean, it's gorgeous. ^_^ (it's not free though.)
Any of those I recommend. Except W.o.W. - don't do that if you are looking for something casual. W.o.W is Meth.